Method for decreasing the thickness of flexible expanded graphite sheet

ABSTRACT

A method for decreasing the thickness of a flexible expanded graphite sheet is provided that includes the steps of providing a a flexible expanded graphite sheet having a surface adhered to a substrate, pulling apart the sheet and the substrate with a force sufficient to separate the adhered flexible expanded graphite sheet into a removed layer and a remainder layer adhered to the substrate; and optionally repeating the foregoing steps until the remainder layer has a desired thickness.

BACKGROUND OF THE INVENTION

[0001] Flexible expanded graphite is a well-known material used in avariety of industrial, commercial and domestic applications because ofits chemical inertness and unique anisotropic electrical and thermalconduction properties. Applications for flexible expanded graphiteinclude chemical and automotive gasketing, valve stem and pump packings,seals, electromagnetic and thermal radiation shielding, furnace linings,heat sinks, and the like.

[0002] Flexible expanded graphite is produced from naturally occurringgraphite flake that is chemically treated to form a compoundintercalated with and between layers of the graphite structure. The“intercalated” graphite particles are then exposed to high temperaturefor a short period of time. The result is an over 80-fold expansion inthe volume between the graphite layers. This expansion (“exfoliation”)produces worm-like or vermiform structures with dendritic rough surfacesthat can then be compressed into sheet material. The density andthickness of the sheet material can be varied by controlling the degreeof compression. For example, the density of the compressed sheetmaterial can range from about 5 to about 137 lbs./ft³, which is near thetheoretical density of graphite. For practical applications, such asthose described above, flexible expanded graphite foil sheet iscommercially available in densities ranging from 50 to 90 lbs./ft³ andthicknesses of 3 to about 60 mils, with a thickness of 15 mils the mostcommon.

[0003] Flexible expanded graphite sheet has a relatively highresistivity along its length and width, and excellent heat conductingand electrical conducting properties that are well suited for use in lowvoltage heater applications. However, the usefulness of this materialfor high voltage heater applications (e.g., 110, 220 or 440 voltsalternating current, VAC) has been limited because of the unavailabilityof flexible expanded graphite heating elements having a sufficientlyhigh electrical resistance.

[0004] A variation in the length, width and/or thickness of flexibleexpanded graphite sheet can change, by a large magnitude, the electricalresistance and, consequently, the amount of electric current that willflow through the material. For a given length and width, an increase inthe thickness of a flexible expanded graphite sheet results in adecrease in the electrical resistance and a higher current flow. Forhigh voltage applications that require very high resistance, it istherefore desirable to use a flexible expanded graphite heating elementthat is as thin as possible. However, commercially produced flexibleexpanded graphite sheet that has a minimum thickness of 3 mils does notprovide a sufficiently high electrical resistance for most high voltageheater applications.

SUMMARY OF THE INVENTION

[0005] Unexpectedly, it has been discovered that the thickness of aflexible expanded graphite sheet of any density can be substantiallydecreased to any desired thickness by methods that do not rely oncompression. Moreover, as starting material, a flexible expandedgraphite sheet of any available thickness and density can be used in themethod for producing a thinner flexible expanded graphite sheet.Ultra-thin flexible expanded graphite sheet (e.g., having a thickness ofabout 2 mils or less) produced by the methods of the invention isparticularly useful as, but not limited to, an electrical resistanceheater element or an electrical strip heater in high voltage heaterapplications.

[0006] In one embodiment, the invention provides a method for decreasingthe thickness of a flexible expanded graphite sheet, comprising thesteps of: (a) providing a flexible expanded graphite sheet having asurface adhered to a substrate; (b) pulling apart the sheet and thesubstrate with a force sufficient to separate the adhered flexibleexpanded graphite sheet into a removed layer and a remainder layeradhered to the substrate; and (c) optionally repeating steps (a) and (b)until the remainder layer has a desired thickness.

[0007] In another embodiment, the invention provides a method fordecreasing the thickness of a flexible expanded graphite sheet,comprising the steps of: (a) providing a flexible expanded graphitesheet having a top surface, and a bottom surface adhered to a firstsubstrate; (b) adhering a second substrate to the top surface; (c)separating the first and second substrates with a force sufficient toseparate the flexible expanded graphite sheet into a first remainderlayer adhered to the first substrate and a second remainder layeradhered to the second substrate; and (d) optionally repeating steps (a),(b) and (c) until a remainder layer has a desired thickness.

[0008] In all the embodiments described above, the resulting remainderlayer of flexible expanded graphite sheet can have a substantiallyuniform thickness, particularly if the second substrate is uniformlyadhered to the top surface. A combination of the above embodiments canalso be employed to obtain a remainder layer having a desired thickness.

[0009] In another embodiment, a method according to the inventionproduces a layer on a substrate of flexible expanded graphite sheet thatis non-uniform in thickness. A method for non-uniformly decreasing thethickness of a flexible expanded graphite sheet, comprises the steps of:(a) providing a flexible expanded graphite sheet having a top surface,and a bottom surface adhered to a first substrate; (b) non-uniformlyadhering a second substrate to the top surface; (c) separating the firstand second substrates with a force sufficient to separate the flexibleexpanded graphite sheet into a first remainder layer adhered to thefirst substrate and a second remainder layer adhered to the secondsubstrate; and (d) optionally repeating steps (a), (b) and (c) until aremainder layer has a desired non-uniform thickness.

[0010] A combination of this method with the above-described embodimentthat does not employ a second substrate can also be used to obtain asheet that is non-uniform in thickness.

[0011] A thin or ultra-thin flexible expanded graphite sheet produced bya method of the invention is adhered to a substrate and can have anydesired thickness, and can retain the density of the original flexibleexpanded graphite sheet. Optionally, the remainder layer produced by anyof the methods can be pressure rolled as a finishing process torecompress the dendritic surface of the graphite to provide surfaceuniformity. If desired, the remainder layer can be sufficientlycompressed by rolling or pressing to produce a thinner and densergraphite.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 illustrates a “one-layer” method for decreasing thethickness of a flexible expanded graphite sheet.

[0013]FIG. 2 illustrates a “two-layer” method for decreasing thethickness of a flexible expanded graphite sheet.

[0014]FIG. 3 illustrates a flexible expanded graphite sheet produced bythe one-layer method or the two-layer method, or a combination of themethods, and having a substantially uniform thickness.

[0015]FIG. 4 illustrates a method for producing a flexible expandedgraphite sheet having a non-uniform thickness.

[0016]FIG. 5 illustrates a flexible expanded graphite sheet having anon-uniform thickness, produced by a method according to FIG. 4.

[0017]FIG. 6 illustrates another embodiment of a flexible expandedgraphite sheet having a non-uniform thickness, that can be produced bythe two-layer method or a combination of the one-layer method and thetwo-layer method.

[0018]FIG. 7 illustrates the effect of various thicknesses on theelectrical resistance of a 4×18 inch flexible expanded graphite foilsheet.

[0019]FIG. 8 illustrates the relationship between the electricalresistance of a 0.2 mils thickness flexible expanded graphite foil sheetand the length and width of the sheet.

DETAILED DESCRIPTION OF THE INVENTION

[0020] Flexible expanded graphite is produced from natural graphiteparticles that are made up of layer planes of hexagonal arrays of carbonatoms that are substantially flat and oriented so as to be substantiallyparallel and equidistant to one another. Natural graphite can thereforebe characterized as laminated structures consisting of superposed layersof carbon atoms joined together by weak van der Waals forces and with ahigh degree of orientation. In considering the graphite structure, twodirections are usually noted, the “c” axis or direction and the “a” axesor directions. The “c” axis is the direction perpendicular to the carbonlayers; the “a” axes are the directions parallel to the carbon layers. Aprocess for producing flexible expanded graphite from natural graphiteis disclosed, for example, in U.S. Pat. No. 3,404,061.

[0021] Briefly, particles of natural graphite are treated with anintercalant of, e.g., a solution of sulfuric and nitric acid, and theintercalated graphite particles are then exposed to high temperature(e.g., greater than 700° C.). Upon this exposure, the spacing betweenthe superposed carbon layers is appreciably opened up so as to provide amarked expansion in an accordion-like fashion to at least 80 or moretimes its original volume in the “c” direction (thickness) dimension.The voluminous expansion of the graphite, without the use of a binder,is believed to be possible due to the excellent mechanical interlocking,or cohesion that is achieved between the graphite particles. Theexfoliated graphite particles are wormlike or vermiform in appearance.The worms are then compressed and subsequently can be roll pressed intoa densely compressed flexible graphite sheet or foil of desired densityand thickness and substantially increased anisotropy with respect toelectrical resistivity and other physical properties. The flexibleexpanded graphite sheet is essentially pure graphite, typically wellover 90 percent carbon by weight, with a highly aligned structure. Onlynaturally occurring minerals remain as part of the product in the formof metal salts, usually referred to as ash.

[0022] It is conventional to compress the exfoliated worms in stageswith the product of the first or early stages of compression being aflexible graphite mat having a density of about 3 to 10 lbs./ft³ and athickness of from 0.1 to 1 inch. The flexible graphite mat can furtherbe compressed by roll pressing into a standard density sheet or foil ofpreselected thickness. A flexible graphite mat thus can be compressed toa thin sheet of between 2 and 180 mils in thickness. The sheet materialcan be die cut into sizes and shapes suitable for a variety ofapplications. The density and thickness of the sheet material can bevaried by controlling the degree of compression. For example, thedensity of the rolled sheet material can range from about 5 lbs./ft³ toabout 137 lbs./ft³, which is near the theoretical density of graphite.

[0023] Commercially available flexible expanded graphite sheet has adensity of 50 to 90 lbs./ft.³, and a thickness of 3 mils to about 60mils. The sheet has a thermal conductivity along the length and width ofabout 140 W/M° K at 21° F. and about 5 W/M° K through the thickness. Theelectrical resistivity of flexible expanded graphite foil sheets havingdensities of 50 to 90 lbs./ft.³ and thickness in a range of 5 mils to 30mils were measured previously and found substantially to comprise valuesof 2.0-6.7×10⁻⁴ ohm-in., as disclosed in U.S. Pat. Nos. 6,237,874 and6,279,856, the disclosures of which relating to the resistivity of thesheets is hereby incorporated by reference.

[0024] The methods of the invention for decreasing the thickness of aflexible expanded graphite sheet take advantage of the laminatedstructure of the sheet that makes it possible to peel or pull awaylayers of the sheet. The flexible expanded graphite sheet that is to bedecreased in thickness can have any original thickness, including athickness of 3 mils to about 60 mils, such as foil sheets that arecommercially available; however, the original thickness can be greaterthan 60 mils, such as 60 mils to about 180 mils. The original thicknesscan also be less than 3 mils, such as about 2 mils. However, thisthickness is not commercially available.

[0025] In one embodiment of the invention, the “one-layer” methodillustrated in FIG. 1, a flexible expanded graphite sheet 1 having asurface adhered to a substrate 2 is provided. To decrease the thicknessof the sheet 1, the sheet and the substrate are pulled apart with aforce sufficient to separate the adhered flexible expanded graphitesheet into a removed layer 3, and a remainder layer 4 that remainsadhered to the substrate 2. The thickness of the remainder layer 4 canvary from about 5% to about 95% of the original thickness of the foilsheet 1, preferably about 25% to about 75%, more preferably about 40% toabout 60% and, especially about 50% of the original thickness.Optionally, the steps of the method can be repeated as often asnecessary, with each successive remainder layer serving as the“original” flexible expanded graphite sheet having a thickness and asurface adhered to the substrate, until a desired thickness of theremainder layer that is less than the thickness of the original sheet isobtained. Thus, regardless of the thickness and density of the originalflexible expanded graphite sheet 1, the thickness of the final remainderlayer can be any desired thickness.

[0026] Another embodiment of the invention, the “two-layer” method, isillustrated in FIG. 2. In this method, a flexible expanded graphitesheet 10 is sandwiched between and adhered to two substrates 11 and 12.That is, the sheet 10 has a bottom surface adhered to a first substrate11 and a top surface adhered to a second substrate 12. To decrease thethickness of the sheet 10, the two substrates 11 and 12 are separatedwith a force sufficient to separate the flexible expanded graphite sheet10 into a first remainder layer 13 that remains adhered to the firstsubstrate 11, and a second remainder layer 14 that remains adhered tothe second substrate 12.

[0027] The thicknesses of each of the remainder layers 13 and 14 areindependent of each other and variable. Usually, the thicknesses of theremainder layers 13 and 14 are independently about 5% to about 95% ofthe original thickness of the foil sheet 10, preferably about 25% toabout 75% and, more preferably, about 40% to about 60% of the originalthickness, especially about 50% of the original thickness. Optionally,the steps of the method can be repeated as often as necessary, with eachof the remainder layers 13 and 14 serving independently as the“original” flexible expanded graphite sheet having a thickness and abottom surface adhered to a substrate. Thus, by adhering a new substrateto a top surface of a remainder sheet, and repeating the separation ofthe substrates, two subsequent remainder layers are obtained, eachthinner than the original remainder layer. The steps of the method canbe repeated until a desired thickness of a remainder layer adhered to asubstrate is obtained. Moreover, each of the remainder layers 13 and 14,or any of the subsequent remainder layers obtained by the two-layermethod, can serve independently as the “original” flexible expandedgraphite sheet in the one-layer method. Correspondingly, a remainderlayer obtained in the one-layer method can serve independently as theoriginal flexible expanded graphite sheet in the two-layer method.

[0028] Regardless of the thickness of the original flexible expandedgraphite sheet the thickness of the flexible expanded graphite sheetlayer can be decreased to any desired thickness by the one-layer method,the two-layer method, or combinations of the methods.

[0029] In the methods of the invention, the step of providing the sheethaving a surface adhered to a substrate can include the substep ofadhering a flexible expanded graphite sheet to a substrate. Moreover,the sheet having a surface adhered to a substrate optionally can have anedge not adhered to the substrate, so as to provide a convenient meansfor engagement of the foil and/or the substrate for applying the pullingforce. However, the presence of an unadhered edge of the sheet is notcritical to the methods of the invention.

[0030] The force sufficient to pull apart the sheet and the substrate,and/or to separate the two substrates, varies with the density of theoriginal adhered flexible expanded graphite sheet, a greater densityrequiring more force. A sheet adhered to a substrate can be separatedinto a removed layer and a remainder layer by manually pulling a layerof the foil away from the adhered foil, or by manually separating thetwo substrates adhered to the foil. For denser sheets, if more forcethan manual separation force is required, a suitable mechanical deviceto engage the foil and/or the substrate and exert a sufficientseparating force can be used.

[0031] Optionally, the remainder layer produced by any of the methodscan be pressure rolled as a finishing process to recompress thedendritic surface of the graphite to provide surface uniformity. Ifdesired, the remainder layer can be sufficiently compressed by rollingor pressing to produce a thinner and denser graphite.

[0032] The substrates employed in the embodiments of the methods can beused independently of each other, especially in methods employing morethan one substrate. For example, the first substrate to which a flexibleexpanded graphite sheet is adhered can be the same as or different thanone or more additional substrates employed in a method embodiment.Suitable substrates for use in embodiments of the method can includesubstantially electrically non-conductive or electrically conductive,flexible or non-flexible substrates. An ultra-thin flexible graphitesheet remainder layer can be adhered to an electrically non-conductivesubstrate. Alternatively, an electrically conductive substrate to whichthe sheet is adhered can be further adhered to an electricallyinsulating material.

[0033] Electrically non-conductive substrates can include, but are notlimited to, polymers such as polyethylenes, polyvinyl chloride,cellulose derivatives, vinyl resins, polystyrenes, polyamides,polyimides, polycarbonates, and acrylic resins such as polymethylmethacrylate, and the like; paper; rubber; mica tape, plate or sheet;glass cloth; glass articles; ceramics such as alumina, mullite, spinel,forstserite and ceramic fiber scrim; and the like. Polymer substratescan be employed in the form of a film. The thickness of the film is notcritical to the methods according to the invention. Exemplary suitableelectrically non-conductive substrates include, but are not limited to,Kapton®, Nomex® and Mylar® (E. I. duPont de Nemours Co.).

[0034] Suitable electrically conductive substrates include, but are notlimited to, metal rods, bars, sheets, foils (e.g., aluminum, magnesium,copper, molybdenum, iron, nickel, silver and titanium), and the like.

[0035] Adhering of the flexible expanded graphite sheet to a substratecan be by mechanical, chemical or thermal bonding. For example,conventional organic or inorganic adhesives can be employed and caninclude rubber cements, animal glues, sodium silica solutions, epoxyresins, phenol formaldehyde resin, and the like, or other commonlyavailable cementing or bonding agents. Such adhesives are particularlyuseful for adhering flexible expanded graphite sheet to electricallyconductive substrates, such as metals, and can be used for virtually anyof the electrically non-conductive substrates. Many polymer films havinga sticky adhesive substance already applied are commercially availableand are well known to those skilled in the art. A convenient method forapplying a thin essentially continuous coating of graphite onto asubstrate is to first coat the substrate with a tacky adhesive orbonding agent, and then to rub or press the flexible expanded graphiteonto the adhesive layer. The coating of the adhesive or bonding agentcan be continuous or non-continuous. It is possible to deposit a layerof adhesive that is continuous in a plane by coating or spraying.However, it can be preferable to have a layer of adhesive that isdiscontinuous but regularly distributed, such as in the form of a grid,or any other pattern including stripes, serpentine pattern, and thelike.

[0036] When the polymer substrates do not have useful adhesionproperties, the bonding between the surface of the flexible expandedgraphite and the substrate can be accomplished by the use of thermalbonding. For example, a thermoplastic polymer film can be fusion-bondedto a sheet of flexible expanded graphite by raising the temperature ofone side of the film to its softening point at the interface engagingthe flexible expanded graphite sheet to cause the film to bond to thesheet, while the opposite side of the film is maintained at atemperature below its softening point. Such methods are well known andone such method for adhering a flexible expanded graphite sheet to asubstrate is disclosed, for example, in U.S. Pat. No. 5,198,063.

[0037] The desired thickness of the thin or ultra-thin flexible expandedgraphite sheet adhered to a substrate as a final remainder layer dependson the application for which it is to be used. Preferably, the thicknessof the sheet is less than 3 mils. For example, the thickness of theinvention sheet can be, but is not limited to, 0.01 mils to 2.5 mils,preferably about 0.01 mils to about 2.0 mils, more preferably about 0.01mils to about 1.5 mils, especially about 0.01 mils to about 1 mils, moreespecially about 0.01 mils to about 0.4 mils, and often about 0.01 milsto about 0.1 mils. Preferably, the thickness of the sheet is suitablefor applications in which a high electrical resistance is desired.

[0038] The thickness of the thin or ultra-thin flexible expandedgraphite sheet adhered to a substrate as a remainder layer can besubstantially uniform or non-uniform. A remainder layer 20 having auniform thickness and adhered to a substrate 21 is illustrated in FIG.3. A non-uniform thickness of the sheet can be useful in manyapplications but is especially useful in heater element applicationswhen it is desired to provide a variation in current through differentparts of the element. A non-uniform sheet preferably is produced by thetwo-layer method, as described below; although it can also be producedby the one-layer method by selectively pulling a layer of the sheet fromthe sheet adhered to the substrate in only desired areas of the adheredsheet.

[0039] To produce a non-uniform sheet by the two-layer method, thesecond substrate is adhered to the first remainder layer 30 adhered tothe first substrate 31 in a discontinuous manner, as illustrated in FIG.4. For example, an adhesive can be applied in the form of a pattern onthe first remainder area and the second substrate 32 is then adhered tothe top surface of the remainder sheet at the points of contact with theadhesive. As illustrated in FIG. 5, when the second substrate and firstsubstrate are separated from each other, the layers of the flexibleexpanded graphite are separated, forming first and second remainderlayers 33 and 34 having patterns of non-uniform thicknesses that aresubstantially mirror images.

[0040] A non-uniform thickness of the sheet can be useful in heaterelement applications when it is desired to provide a variation incurrent through different parts of the element. As illustrated in FIG.6A an adhesive and second substrate 40 can be applied in selected areasof the original and/or remainder layers 41 adhered to the firstsubstrate 42. Separation of the substrates removes a layer of theoriginal/remainder 43 resulting in a pattern, such as that illustratedin FIG. 6B. An adhesive and third substrate 44 can then be applied toonly selected areas of the remainder layer 45. This can be repeated, ifdesired, to form shoulder areas 46, such as those illustrated in FIG.6C. It is envisioned that any pattern or shape of non-uniformthicknesses can be obtained by this method. Alternatively, a combinationof the two-layer method and the one-layer method can producesubstantially the same results.

EXAMPLES

[0041] The following examples illustrate certain advantages ofultra-thin flexible expanded graphite sheets, produced by embodiments ofthe methods of the invention, as resistance heater elements for highvoltage applications. However, the examples are not intended to belimiting, as other applications for the ultra-thin sheets can readily bedetermined by those skilled in the art. The examples have been providedmerely to demonstrate the practice of the subject invention and do notconstitute limitations of the invention. Those skilled in the art canreadily select other wattages, voltages, lengths, widths andthicknesses, and the like, according to the disclosure made hereinabove. Thus, it is believed that any of the variables disclosed hereincan readily be determined and controlled without departing from thescope of the invention herein disclosed and described.

[0042] The design and manufacture of resistance heating elements dependson the wattage required for a given application, and the correspondingresistance determined by Ohm's Law which states that, for any circuit,the electric current is directly proportional to the voltage and isinversely proportional to the resistance.

[0043] By Ohm's Law:

Power(watts)=I ²(amps)×R(ohms)

Power=I×V(voltage)

I=V/R and R=V/I

Power=V ² /R

[0044] where I=Current, R=Resistance and V=Voltage.

[0045] The required resistance of a flexible expanded graphite foilsheet having a given length and width, used as a heating element,determines the thickness requirement of the sheet. It is known that theelectrical resistivity of the foil sheet varies with the density, andthe resistance along a given length and width of the foil sheet varieswith the thickness. Therefore, the required thickness of flexibleexpanded graphite foil sheet having a known resistivity, density, length(L) and width (W) can be calculated, as follows, where A is thecross-sectional area (W×thickness, t) of the foil sheet:

L/A×volume resistivity=resistance (R)

Example 1

[0046] A flexible expanded graphite foil sheet having a volumeresistivity of 3.4×10⁻⁴ ohm-in. and a length and width of 18 in. and 4in., respectively, is used as a heating element in a heater requiring 10watts/in.² and run at 110 VAC. The total wattage is 720 watts. From theabove equations:

[0047] 720=I×E where E=110 volts

[0048] I=6.54 amps

[0049] 720=(6.54)2×R

[0050] R=16.81 ohms

[0051] 18/A×3.1×10⁻⁴=16.81

[0052] A=(18×3.1×10⁻⁴)/16.81

[0053] A=0.00033 in²

[0054] A=W×t

[0055] t=A/W

[0056] t=0.08 mils

[0057] When the dimensions of the flexible expanded graphite foil arechanged to 36 in.×2 in., in the same scenario as above, the requiredthickness is

[0058] A=(36×3.1×10⁻⁴)/16.81

[0059] A=0.00066

[0060] t=0.33 mils

[0061] When the voltage is increased to 220 VAC and the dimensions ofthe flexible expanded graphite foil are 36 in.×2 in, in the samescenario as above, the required thickness is:

[0062] 720=I×E where E=220 volts

[0063] I=3.27 amps

[0064] 720=(3.27)2×R

[0065] R=67.22 ohms

[0066] 36/A×3.1×10⁻⁴=67.22

[0067] A=(36×3.1×10⁻⁴)/67.22

[0068] A=0.00016 in²

[0069] A=W×t

[0070] t=A/W

[0071] t=0.08 mils

[0072] Similarly, using a voltage of 440 VAC and a dimension of theflexible expanded graphite sheet of 72 in.×1 in., the required thicknessis t=0.08 mils.

Example 2 (Comparison Example)

[0073] A flexible expanded graphite foil sheet having a length and widthof 18 in. and 4 in., respectively, and a thickness of 3 mils, isproposed for use as a heating element in a heater requiring 10watts/in.². The total wattage is 720 watts.

[0074] From the above equations, the resistance of the heating elementis 0.465 ohms. According to the calculations, when 110 VAC is applied tothis heating element, the flexible expanded graphite heater will useabout 237 amps and produce about 26,022 watts, both unrealistic numbers.Therefore, a flexible expanded graphite heating element having athickness of 3 mils does not produce a sufficiently high resistance foruse in this high voltage application.

Example 3

[0075] A flexible expanded graphite sheet having a density of 70lbs./ft³, a length and width of 18 in. and 4 in., respectively, and athickness of 5 mils, was adhered to a Kapton® Type HN film having athickness of 2 mils, by an adhesive silicone coating having a thicknessof 1.0 mils.

[0076] A layer of the flexible expanded graphite sheet was manuallypulled away from the film substrate, leaving a remainder layer of thesheet adhered to the film. The thickness of the remainder layer and thefilm substrate was measured with a micrometer and the thickness of theremainder layer was determined to be 2.5 mils. A second piece of thefilm was then uniformly adhered to the exposed surface of the remainderlayer. The film substrates were then manually separated with a manualforce sufficient to separate the remainder layer of the flexibleexpanded graphite into two new remainder layers, one adhered to eachfilm. The thicknesses of the layers were determined to be 1.2 mils and1.3 mils, respectively.

[0077] A third film was similarly adhered to the 1.2 mils remainderlayer and the manual separation of the substrates was again performed,resulting in remainder layers of 0.5 mils and 0.6 mils. The 0.5 milsremainder layer was then similarly adhered with a fourth piece of filmand the manual separation of the substrates was performed. The resultingremainder layers were both 0.25 mils in thickness. The method was thenrepeated, resulting in remainder layers of 0.1 mils in thickness.

[0078] Similarly, selected remainder layers having various thicknessesdescribed above were treated by the one-layer method and/or thetwo-layer method until separate remainder layers having thicknesses of0.1 mils, 0.75 mils, 1.5 mils, respectively, were obtained.

[0079] The resistance in ohms of the respective 18 in.×4 in. ultra-thinflexible graphite foils adhered to the respective remainder layers wascalculated by known methods, using the equations described above andvoltmeter and ammeter measurements and compared to the resistance of a 3mils commercially available flexible expanded graphite foil adhered tothe same substrate in the same manner as described above.

[0080] The results, shown in FIG. 7, illustrate the significant increasein resistance that is achieved by decreasing the thickness of the foilby the methods of the invention. For example, the foil having athickness of 0.1 mils has a resistance that is more than 22 times theresistance of the comparison 3 mils foil. Moreover, the resistance ofthe 1.5 mils foil and the 0.75 foil is about 2 times and about 4 times,respectively, that of the comparison 3 mils foil.

Example 4

[0081] The method of Example 3 was repeated for individual flexibleexpanded graphite foil sheets having a density of 70 lbs./ft³, athickness of 5 mils, and various combinations of lengths ranging from 5to 60 inches, and widths ranging from 0.2 to 0.5 inches. Remainderlayers of the foil were obtained for each length and width combinationand having a thickness of 0.2 mils. The resistance in ohms was measuredfor each foil layer, and the results are illustrated in FIG. 8. Theresults show the significant increase in resistance of the ultra-thinflexible expanded graphite foil sheet having a given thickness, as thelength increases and/or width of the sheet decreases. The resistance ofthe ultra-thin flexible expanded graphite sheet, produced according tothe methods of the invention, can thus be selected by the selecting theproper thickness, length and width of the remainder material.

[0082] This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and can include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have elements that do not differ fromthe literal language of the claims, or if they include equivalentelements with insubstantial differences from the literal language of theclaims.

We claim:
 1. A method for decreasing the thickness of a flexibleexpanded graphite sheet, comprising the steps of: (a) providing aflexible expanded graphite sheet having a surface adhered to asubstrate; (b) pulling apart the sheet and the substrate with a forcesufficient to separate the adhered flexible expanded graphite sheet intoa removed layer and a remainder layer adhered to the substrate; and (c)optionally repeating steps (a) and (b) until the remainder layer has adesired thickness.
 2. The method of claim 1, wherein the thickness ofthe remainder layer is substantially uniform.
 3. The method of claim 1,wherein the thickness of the remainder layer is non-uniform.
 4. A methodfor decreasing the thickness of a flexible expanded graphite sheet,comprising the steps of: (a) providing a flexible expanded graphitesheet having a first thickness, a top surface, and a bottom surfaceadhered to a first substrate; (b) adhering a second substrate to the topsurface; (c) separating the first and second substrates with a forcesufficient to separate the flexible expanded graphite sheet into a firstremainder layer adhered to the first substrate and a second remainderlayer adhered to the second substrate; and (d) optionally repeatingsteps (a), (b) and (c) until a remainder layer has a desired thicknessthinner than the first thickness.
 5. The method of claim 4, wherein thethicknesses of the remainder layers are independent and aresubstantially uniform.
 6. The method of claim 4, wherein the thicknessof the remainder layers are independent and are non-uniform.
 7. A methodfor non-uniformly decreasing the thickness of a flexible expandedgraphite sheet, comprising the steps of: (a) providing a flexibleexpanded graphite sheet having a top surface, and a bottom surfaceadhered to a first substrate; (b) non-uniformly adhering a secondsubstrate to the top surface; and (c) separating the first and secondsubstrates with a force sufficient to separate the flexible expandedgraphite sheet into a first remainder layer adhered to the firstsubstrate and a second remainder layer adhered to the second substrate;and (d) optionally repeating steps (a), (b) and (c) until a remainderlayer has a desired non-uniform thickness.
 8. The method of claim 7,wherein the thicknesses of the remainder layers are independentlynon-uniform.
 9. A thin flexible expanded graphite sheet, produced by aprocess comprising the steps of: (a) providing a flexible expandedgraphite sheet having a first thickness and a surface adhered to asubstrate; (b) pulling apart the sheet and the substrate with a forcesufficient to separate the adhered flexible expanded graphite sheet intoa removed layer and a remainder layer adhered to the substrate; and (c)repeating steps (a) and (b), if necessary, until the remainder layer hasa desired thickness that is thinner than the first thickness.
 10. Thesheet of claim 9, wherein the thickness of the remainder layer issubstantially uniform.
 11. The sheet of claim 9, wherein the thicknessof the remainder layer is non-uniform.
 12. A thin flexible expandedgraphite sheet, produced by a process comprising the steps of: (a)providing a flexible expanded graphite sheet having a first thickness, atop surface, and a bottom surface adhered to a first substrate; (b)adhering a second substrate to the top surface; (c) separating the firstand second substrates with a force sufficient to separate the flexibleexpanded graphite sheet into a first remainder layer adhered to thefirst substrate and a second remainder layer adhered to the secondsubstrate; and (d) repeating steps (a), (b) and (c), if necessary, untila remainder layer has a desired thickness thinner than the firstthickness.
 13. The sheet of claim 12, wherein the thicknesses of theremainder layers are independent and are substantially uniform.
 14. Thesheet of claim 12, wherein the thicknesses of the remainder layers areindependent and are non-uniform.
 15. A thin flexible expanded graphitesheet having a non-uniform thickness, produced by a process comprisingthe steps of: (a) providing a flexible expanded graphite sheet having atop surface, and a bottom surface adhered to a first substrate; (b)non-uniformly adhering a second substrate to the top surface; (c)separating the first and second substrates with a force sufficient toseparate the flexible expanded graphite sheet into a first remainderlayer adhered to the first substrate and a second remainder layeradhered to the second substrate; and (d) optionally repeating steps (a),(b) and (c) until at least a portion of a remainder layer has athickness of about 0.01 mils to less than about 0.5 mils.
 16. The sheetof claim 15, wherein the thicknesses of the remainder layers areindependently non-uniform.